A: The term "jet stream describes the ribbons
of fast-moving air located five to eight miles above the Earth's surface. The
jet stream was first discovered by meteorologist Wasburo Ooishi in the 1920s
during his studies of high-elevation wind patterns over Japan using weather
balloons, but it wasnt until 1939 that German meteorologist H. Seilkopf
used the term "jet stream" in a published scientific paper. American
aviation pioneer Wiley Post is often credited with first flying within a jet
stream.

Q: Does the temperature of
air masses affect the speed and location of jet streams?

A: Yes, jet streams are simply rivers of fast-moving winds aloft in the atmosphere.
Jet streams form along the upper-air boundaries of large masses of warm and
cold air. Wind speed is dependent upon differences in air pressure, which are,
in turn, dependent upon temperature differences. The larger the temperature
difference, the tighter the pressure gradient and the stronger the wind speed.

Q: How many jet streams are there? I've heard
of the polar jet and the subtropical jet (along with "the southern branch"
of the polar jet.) Are there others?

A: The American Meteorological
Society (AMS) Glossary of Meteorology defines the term jet stream as "relatively
strong winds concentrated within a narrow stream in the atmosphere."

Although the most commonly referred-to
jet streams are the polar front jet (which typically occurs at latitudes between
50°and 60°, both north and south of the equator) and the subtropical
jet (around 20° to 30°), a jet stream can technically occur anywhere
in the upper atmosphere (typically, 6-9 miles above the Earth's surface) when
wind speeds exceed 50 knots, or about 58 mph. Wind speeds in jet streams average
about 110 mph, although they can reach as high as 300 mph. Another example is
the tropical easterly jet, which occurs between India and the equator during
July and August.

In addition to these high-altitude
"rivers" of air, forecasters often look for low-level jets, which
occur at lover altitudes and travel at slower speeds than their upper-level
counterparts.

Q: Does the jet stream create low- and high-pressure
areas, or is it the other way around?

A: The jet
stream is often described as a fast-moving river of air that forms along
the boundaries of the cold and warm air masses that surround the Earth. The
greater the temperature contrast, the stronger the jet stream.

Low-pressure
areas (storms) form along these boundaries and are often near the jet streams.
These jets and lows are really waves in our ocean of air and like other waves
can interact, amplify and strengthen. Storms don't create the jet stream, but
a strong jet stream can help create a very powerful storm.

A: The Gulf Stream is a warm surface current in the
Atlantic Ocean that runs from the Gulf of Mexico to the northwest coast of Europe.
Although its direct effects on New Englands weather are likely small,
it does help form and strengthen the nor'easters
that sometimes batter the area in winter. The contrast between cold air over
land and the warm Gulf Stream water off the Carolina coast help these winter
storms develop.

The Gulf Stream clearly influences the climate of
northwestern Europe, which experiences much warmer temperatures than places
that lie on the same latitude along the New England coast.

Q: Why do the jet streams travel from west to
east in both hemispheres? If the three-cell model of the general circulation
of the atmosphere is correct, wouldn't it predict that wind at altitude would
travel in an east to west direction at mid latitudes.

A: First, for those who don't know, the "three cell"
model refers to the very general, global pattern of winds. It includes:

Hadley cells in the tropics with air rising near
the equator, sinking at around 30 degrees north and south, and flowing back
along the surface toward the equator.

Ferrel cells in the middle latitudes with some
air flowing along the surface from around 30 degrees toward the poles.

Polar cells with air on the surface flowing toward
the equator to around 60 degrees north or south, where it rises.

This model does a pretty good job of explaining the
general flow of winds at the Earth's surface. Air flowing toward the equator
in the Hadley cells is turned to the right, or from the east, by the Coriolis
force in the Northern Hemisphere and to the left, also from the east, south
of the equator. This creates the easterly trade winds. The model also nicely
explains westerly surface winds in the middle latitudes of both hemispheres
and polar easterlies in the Arctic and Antarctic.

But, as you point out, there is something wrong with
the idea of the upper air flow in the middle-latitude Ferrel cell moving toward
the equator, which is what the model shows. Winds heading for the equator in
both hemispheres are turned toward the west by the Coriolis force. Yet, we know
that winds above the middle latitudes would have to be blowing from the equator
toward the poles to create the jet streams blowing from the west.

Your question is a good one; the kind that led to
a discussion among those on the USA TODAY Weather Team. We agree that the best
answer is that the three-cell model is too simple to capture what's really going
on in the middle latitudes. As you probably know, the main thing going in the
middle latitudes to mess up the simple picture of the three-cell model is the
extratropical
storms that keep the atmosphere stirred up. These storms carry air both
north and south. You could think of the southward flow of upper air shown in
the model as representing the average of this flow and also flow by jet streams
when they head south, not as an actual picture of all that's going on.

The jet
streams blowing from the west begin with higher pressure aloft, say at 30,000
feet, above areas of warm air. The pressure difference begins pushing winds
toward the area aloft above cold air. Generally, the warm air is on the equator
side and the cold air on the pole side. As the air moves toward the pole the
Coriolis force turns it to the right in the Northern Hemisphere and to the left
in the Southern Hemisphere. In both cases, this creates jet streams flowing
generally from west to east. Jet streams are like the caps, or tops, of boundaries
between warm and cold air.

You won't find the details of how this works on the
Web. You can start with Chapter 3 of my USA
TODAY Weather Book. For more details, a good introductory meteorology textbook
such as Meteorology Today by C. Donald Ahrens, is the best bet. (12-2-98)

Q: Do the high and low pressure systems affect where
the jet stream runs, or does the jet stream affect where these systems will go?

A: This is one of those "which came first, the chicken
or the egg" questions. Briefly, jet streams are located over boundaries of warm
and cold air. A USA TODAY Online graphic shows how
this works. You'll find links to more about jet streams at the bottom of
the text with the graphic. But, jet streams also help determine the locations
of high and low pressure areas, which in turn help create flows of warm and
cool air that set up the boundaries. The links from the graphic will lead you
to several other graphics on these topics. Also, chapters 3 and 4 of my USA
TODAY Weather Book has a lot more detail about all of this (10-18-98)

Q: Hi, I am doing a report on jet streams for
school and I need to find out the elevation of the jet stream over a 19 day
period. Do you know where I could find this information?

A: Finding the elevation of jet streams will be pretty
difficult unless you know how to read upper-air charts, or are willing to learn
some of the basics. Jet streams are narrow rivers of strong winds, usually faster
than 55 mph in the general flow of upper-air winds. Normally you find them on
the charts showing what's happening at the 300 millibar level, which is about
30,000 feet above sea level. You want to find the charts called the 300 mb analysis
(In weather talk an analysis map is one showing the actual conditions at a particular
time). The full versions of 300 mb charts give the height above sea level of
the 300 mb pressure at weather stations where weather balloons took the measurements.
Once you learn how to read the maps, you could find a weather station with the
jet stream overhead and see how high it was.

Of course, the 300 mb charts are just a slice of
the atmosphere at that particular pressure level. The fastest jet stream winds
could extend 1,000 or so feet above or below that level. And, sometimes jet
stream winds are found at lower levels.

If you go to our page of links to technical
weather information and scroll down to the "Jet stream maps" headline, you'll
find links to such maps and also to information on how to read them. One of
the links is to a San Francisco State University Web site, which includes an
archive of maps as well as current ones.

I wonder, once you do all of this work, what will
you have? The height of the jet stream doesn't make much difference to the weather.
But, wind speeds, especially changes in speed, and the twists and turns of upper-air
winds are key to the weather. A USA TODAY graphic
on jet streams and the links from it to other graphics and text give the
basics of how jet streams affect the weather. (5-25-98)

Q: I realize that the jet streams play a major
role in the extended weather for a region. Apparently, meteorologists can create
an expected forecast based on, among other things, they flow of the jet stream
and how troughs and ridges are expected to form. I'm interested in finding out
how meteorologists can make jet stream forecasts? Are they based on computer
models? When (or how) do you know that a jet stream will change by the "middle
of next week"? Does the change in jet stream locations progress across the globe?
For example, if you have noticed that the jet stream has changed over the western
Pacific, does that indicate that the jet stream over the Pacific Northwest will
be changing within a few days or weeks?

A: You seem to have a good understanding of weather
forecasting. The position of the jet
stream with its troughs and
ridges plays a major role in extended
weather forecasts. Computer models are used
extensively for jet stream position forecasts. If the jet stream changes in
one location, this change often affects other locations upstream and downstream.
For example, if a large ridge builds in the western Atlantic, the jet stream
often "buckles" over the USA with a large trough forming over the eastern part
of the country. Our weather forecasting homepage
has much more detailed information on how weather forecasts are done including
the different kinds of computer models currently
in operation.

(Answered by Chad Palmer, USA TODAY Weather team)
(4-19-97)

Q: Why does the jet stream usually go from west
to east. Is this related to the rotation of the Earth?

A: You're correct, the general west-to-east flows
of jet streams are related to the Earth's
rotation. In any level of the upper atmosphere, say 30,000 feet, the air pressure
is higher above places where the air is warm than above places where the air
is cold. This means that at 30,000 feet (or any other altitude) the pressure
is higher over the tropical or sub-tropical regions than at the same altitude
over the polar regions. Such pressure differences
cause the winds to start blowing. Once the wind begins moving, the Coriolis
effect, which is caused by the Earth's rotation, changes the wind's direction.

Let's see how this works in both hemispheres. In
the Northern Hemisphere the wind starts blowing from south to north. The Coriolis
effect turns it to the right, making it flow from west to east. South of the
equator, the wind begins blowing from north toward the south. Here the Coriolis
effect turns it to its left. The result is also a west-to-east jet stream in
the Southern Hemisphere. (3-16-97)

Q: Does the jet stream dip thus developing a trough
or does a trough develop and thus draw down the jet stream?

A: The dip in the jet
stream is a trough by definition.
The position of the polar front is the main
factor that determines the position of the jet stream. As the polar front dips
south during cold outbreaks, the jet stream also dips south and forms troughs.

(Answered by Chad Palmer, USA TODAY Weather team)
(2-27-97)

Q: What causes the jet stream to travel south
in the wintertime?

A: Jet stream winds blow along boundaries between
warm and cold air. As cold air advances farther south in the winter it pushes
warm-cold boundaries, and jet streams, south with it. When warm air moves north,
the boundaries and jet streams are pushed back to the north. A USA TODAY online
graphic explains more about how this works and has links to more information
about winds and jet streams. (10-26-96)

Q: Is the jet stream pushed further into the USA
by high pressure to the north, and if so, what is it that ultimately creates
either a high or low pressure area?

A: The jet stream is not really "pushed" into the
USA by an area of high pressure to
the north. In fact jet streams and
jet streaks are a couple of the many factors that determine where high and low
pressure areas form. The jet stream forms along the polar
front and it is the position of the polar front that determines where the
jet stream is located. If the jet stream dives south in a meridional
flow pattern, a cold dome of high pressure is allowed to slide into the
USA and put the country in a deep freeze. However, in a zonal
flow pattern, the cold air is blocked by the jet stream from diving into
the USA. The jet stream is not pushed around by areas of high or low pressure.
Instead, the jet stream often helps determine where lows and highs form. (Answered
by Chad Palmer, USA TODAY Information Network) (4-29-96)

Q: What is the jet stream? How does it form? How
is it different from a front? Does one affect the other?

A: The jet
stream is a high-altitude river of fast-moving air with wind speeds of 57
mph or greater. The two main jet streams that influence the USA's weather are
the polar jet and the subtropical jet stream. A front is a boundary between
different air masses. Warm, cold,
stationary, and occluded
fronts all have different implications for the weather. The jet stream and the
polar front are very closely linked together. The polar front refers to the
main cold front that separates cold, polar to the north from warm, tropical
air to the south. Air movements associated with the polar front lead to the
development of the jet stream. An USA TODAY Online graphic explains more about
the formation of jet streams.

(Answered by Chad Palmer of USA TODAY Information
Network) (3-27-96)

Q: On television I hear the weather term "short
wave" used. Could you give me a brief definition of this term?

A: Let's start with long waves. When you see charts
of the upper air winds or the jet stream, you'll notice that sometimes they
make a wavy pattern with the jet stream coming down over the South and then
back over the North. A USA TODAY Online graphic
shows this and what it means. Anyway, one of these "long waves" in the flow
of upper-air winds can stretch across the U.S. "Short waves" are "kinks" in
these long-wave patterns. While the long waves can stay in place for days, the
short waves travel along with the winds. Such short waves can bring clouds and
rain. They are also called "upper-air disturbances." We have another
online graphic with more on what they do. (3-13-96)